The present invention relates generally to electron emitters, and, more particularly, to an apparatus for modifying an aspect ratio of an electron beam to form a focal spot having a desired size and aspect ratio on a target anode.
In an x-ray imaging system, formation of a small focal spot on a target anode is desired to achieve high-quality x-ray imaging. In order to maximize the target thermal management and cathode emission capability, an x-ray tube is configured such that a non-circular (i.e., linear) focal spot is formed on the target. In the imaging system, the detector will view the x-ray spot at a shallow angle (7-12 degree) to achieve an effective small optical spot size.
To achieve a linear focal spot, today's x-ray tubes use a linear electron emitter element or cathode that has almost the same aspect ratio as the desired focal spot, and a focusing cup/electrode is used to focus the electron beam onto the target anode. The drawback of using such large aspect ratio cathodes is its difficulty of beam optics design. That is, with the generation of an electron beam with a large aspect ratio (by a similarly shaped cathode), it is very difficult to design good beam optics, which are required to achieve a small focal spot on the target.
Additionally, with specific reference to x-ray tubes that implement a thermionic cathode, the large aspect ratio of the cathode imposes additional problems. That is, in order for the thermionic cathode to have a uniform emission, the temperature on the cathode surface has to be uniform. In an x-ray tube environment, a temperature of the cathode surface can be as high as 2600 degrees Celsius, while the surrounding area is at room temperature. For a large aspect ratio thermionic cathode, there is a larger edge area, which results in a temperature of the cathode that tends to be less uniform than a circular cathode.
While the use of linear emitter elements that generate linear electron beams has various drawbacks as set forth above, circular emitter elements that generate circular electron beams can mitigate some of these problems. That is, the use of a circular electron beam allows for the simpler design of beam optics that will generate a small focal spot. Additionally, for emitter elements that are in the form of a thermionic cathode, a circular emitter element profile promotes a stable and consistent temperature thereacross, so as to provide for a more uniform emission of electrons.
Thus, a need exists for an apparatus that provides for the generation of a circular electron beam, so as to allow for the simpler design of beam optics for focusing the electron beam, while still providing for formation of a small, linear focal spot on a target anode.